In the production of molded articles, it is known to supply two reactive resin components, e.g. an isocyanate and a polyol in the case of a polyurethane or an epoxy and a hardener in the case of epoxy resins, to a mixing chamber in which the two flowable reactive components are intimately mixed and discharged from the mixing chamber into a mold in which the synthetic resin composition hardens or sets.
In such systems, e.g. from German open application DE-OS No. 28 37 425, it is known to provide respective supply vessels for the two components and to feed the components to the mixing chamber via dosing or metering cylinders in which respective dosing and metering pistons are displaceable, each of the reactant-displacing pistons being coupled to a working piston displaceable in a working cylinder under hydraulic actuation and for which an appropriate hydraulic control circuit is provided.
In the system of this publication, which is also utilized for production of articles from two component synthetic resin reaction systems, each of the reaction components is fed from a respective supply vessel to the mixing head and from the mixing head can be returned to the supply vessel if the mixing head is of the recirculating type.
The dosing or metering of the respective reactive component is effected by the piston assembly assigned thereto, the displacement rate of the metering piston determining the rate at which the reactive component is positively displaced from the metering cylinder to the mixing head.
The working cylinders of the metering units are connected in a hydraulic circuit so that they are hydraulically energized by respective controllable pumps. A controllable pump, for the purpose of the present description, will be understood to be a pump which is of the variable displacement type, i.e. is driven by a motor but can have the rate of displacement (displacement volume per revolution) varied.
In the system of the type described, a respective pump is provided for each metering cylinder and hence for each component, a common pump for all of the hydraulic circuits being impractical because of the different parameters under which the respective hydraulic circuits operate and especially the different speeds with which the respective working pistons must move to displace the associated reactive component at the proper rate to form the mixture.
This is not to say that it is not known to provide a common pump for a plurality of hydraulic circuits. Indeed, in the earth-moving machine of British Pat. No. 1,126,873, reversible hydraulic pistons which operate the various elements of this machine are supplied by a common pump which has its displacement pressure controlled. In this system the hydraulic circuits are operated by respective manually controlled distributing valves and the controlled pressure for the pump derives from the highest pressure prevalent in a feed line of one of the hydraulic pistons via a check valve.
Mention has been made in the printed British application No. 2,099,610 of an injection-molding machine for synthetic resin materials which has a hydraulic circuit whose mold-closing hydraulic circuit is provided with a volume-control valve controlled by a displacement pickup or a pressure sensor.
In the past a system utilizing separate pumps for respective hydraulic circuits as previously described for producing articles from two component reactive systhetic resin compositions have been used successfully for apparatus with low displacement requirements, e.g. for the production of small articles weighing say 30 to 3,000 grams per piece. However, for larger articles, these systems have proved to be unsatisfactory and in general it has long been desirable to provide in the field of the invention, namely, in the field of molding compositions consisting of reactive synthetic resin components, a way of utilizing a single displacement pump to control the displacement of a plurality of reactive components, each via a respective metering piston and hydraulic circuit in spite of the fact that these pistons are required to move at different rates. Adaptation of these earlier hydraulically-controlled systems to the specific needs of the molding art has not proved successful heretofore to the best of my knowledge.